Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most types of internal combustion engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
A rear wheel drive vehicle 10 is illustrated schematically in a FIG. 1. Solid like indicate the flow of mechanical power while dotted lines indicate the flow of information signals. Engine 12 provides the mechanical power and delivers the power to transmission input shaft 14. Transmission 16 transmits the power to driveshaft 18 while adjusting the speed to match vehicle speed to appropriate engine speed. Differential 20 divides the power between left and right drive wheels 22 and 24 allowing slight speed differences between the wheels as the vehicle turns a corner. In a rear wheel drive vehicle, the differential also changes the axis of rotation by 90 degrees and reduces the speed by a fixed final drive ratio. In a front wheel drive vehicle, the components may be oriented differently, and power may be transferred from the transmission gearbox to the differential by gearing rather than by a shaft.
Controller 26 provides signals to engine 12 to control the level of torque produced. For example, these signals may control the throttle opening, spark timing, etc. Controller 26 also provides signals to transmission 16 to control the speed ratio between the transmission input shaft 14 and the transmission output shaft which is fixed to driveshaft 18. This may include signals that control the torque capacity of clutches within transmission 16 which, when engaged in different combinations, establish different power flow paths with different speed ratios. Controller 26 receives signals from various sensors, such as speed signals from transmission 16 and the position of a driver operated accelerator pedal 28 that indicates a driver demanded torque. Controller 26 may also utilize signals from other sensors such as an accelerometer 30 and a vehicle navigation system 32, such as a system based on Global Positioning System (GPS) satellites.
Controller 26 decides when to command a shift from one gear ratio to another gear ratio. This decision making is called shift scheduling. Although the shift scheduling algorithm may consider many different inputs, the two primary inputs are typically an indication of speed and an indication of torque demand. A representative shift schedule for a four speed automatic transmission is depicted in FIG. 2. The horizontal axis represents vehicle speed or, equivalently, transmission output shaft speed which is proportional to vehicle speed. The vertical axis represents driver demanded output torque which is typically determined based on accelerator pedal position. At any given time, the current operating condition corresponds to a point in this graph. The controller schedules an upshift from 1st gear to 2nd gear whenever the current operating point crosses from the left side of 1-2 upshift line 40 to the right side of line 40. This can occur either because the vehicle increases in speed, because the driver reduces the demanded torque, or a combination of the two. The controller schedules a downshift from 2nd gear to 1st gear whenever the current operating point crosses from the right side of 2-1 downshift line 42 to the left side of line 42. Downshift line 42 is offset slightly from upshift line 40 in order to avoid frequent shifts in response to small changes in speed or demanded torque. Shifts between 2nd gear and 3rd gear are scheduled similarly using 2-3 upshift line 44 and 3-2 downshift line 46. Shifts between 3rd gear and 4th gear are scheduled similarly using 3-4 upshift line 48 and 4-3 downshift line 50. For transmissions with more than four ratios, the shift schedules have more lines, but are conceptually similar.
The location of each shift line is determined during a calibration process. Calibration requires compromises between various criteria. To generate maximum power, an internal combustion engine typically must rotate at high speed. Therefore, at high torque demands, the shift schedule tends to select lower numbered gears such that engine speed is near the power peak. At low torque demands, the schedule selects higher gear numbers such that the engine speed is near the most efficient operating speed for the corresponding power requirement. The location of some shift lines may be altered from a theoretical ideal to reduce the number of shifts requires for particular maneuvers or to satisfy other customer expectations.